The widespread use of mobile terminals such as a cellular phone and a notebook-size personal computer recognizes the importance of secondary batteries acting as their power sources. These secondary batteries are required to be compact, light and high in their capacities, and to have performance hardly deteriorated after the repetition of the charging and discharging.
Although metal lithium may be used as the anode of the secondary battery in view of its higher energy density and lightness, a problem arises that needle crystals (dendrite) are deposited on the lithium surface of the metal lithium anode during the charging in the progress of the charge-discharge cycle, and the crystals penetrate the separator to cause the internal short-circuit, thereby shortening the battery life.
Further, use of lithium alloy having a composition formula of LixM (“M” is metal such as Al) as an anode is investigated. A problem also arises that pulverization of the anode is caused with the progress of the charge-discharge cycle thereby shortening the battery life because the lithium metal anode is swollen and contracted with the insertion and extraction of the lithium ion though it has a larger volume for the lithium ion insertion and extraction per a unit volume and thus has a larger capacity.
When a carbon material such as graphite and hard carbon capable of inserting and extracting the lithium ion is used as the anode, the energy density becomes lower because the graphite material has a lower capacity than the metal lithium and the lithium alloy, and the hard carbon has a larger irreversible capacity on the initial charging and discharging to decrease the charge-discharge efficiency though the charge-discharge cycle is excellent.
A number of investigations have been conducted for the purpose of improving the anode.
JP-A-2000-21392 discloses the electric contact between an anode containing a carbon material and metal such as Si, Ge and Sn or its oxide, and metal lithium during the fabrication of the battery to propose the improvement of the anti-over discharge performance and the cycle performance.
JP-A-11(1999)-135120 discloses use of a carbon material coated with particles made of Al, Sn or Sb as an anode to propose the improvement of the higher capacity, the higher voltage and the cycle performance.
JP-A-10(1998)-21964 discloses use of a material mainly containing chalcogen compounds having Al, Sn or Si or its oxide as an anode to propose the higher capacity, the elevation of the cycle performance and the improvement of the production efficiency.
JP-A-2000-182602 discloses a secondary battery anode including an anode sheet made of an amorphous oxide capable of inserting and extracting lithium and laminated with a metal foil mainly made of lithium to propose the higher capacity and the improvement of the anti-over discharge performance.
JP-A-2001-15172 discloses a secondary battery anode including an anode sheet made of a carbon material laminated with a metal foil mainly made of lithium to propose the higher capacity and the improvement of the charge-discharge efficiency.
However, these prior arts cause the following problems.
The techniques described in JP-A-2000-21392, JP-A-11(1999)-135120 and JP-A-10(1998)-21964 can hardly increase the energy densities of the batteries sufficiently high because the metals and the metal oxides have the higher irreversible capacities on the initial charging and discharging and the larger anode weights. When the metal is mixed with the carbon-based material, the operating voltage becomes lower compared with an anode made of only carbon, and the higher operating voltage can be hardly obtained because a voltage plateau which is typical to the metal appears on a discharge curve at a voltage higher than that of carbon so that the higher operating voltage can be hardly obtained. The lower limit voltages are fixed depending on uses in the lithium secondary battery. The decrease of the operating voltage narrows the usable region so that the capacity increase in the region where the battery is actually used can be hardly intended.
In the method described in JP-A-2000-21392, the added lithium is reacted with active functional groups on the carbon surface, adsorbed water on the carbon surface or moisture contained in the solvent or the electrolyte to form a film on the anode surface. The lithium contained in the film is electrochemically inactive and cannot contribute to the charge-discharge capacity so that the improvement of the charge-discharge efficiency is insufficient. The electric resistances of the films are large to increase the resistance of the battery so that the effective capacity of the battery rather decreases.
In both of the anode sheet made of the amorphous material and the anode sheet made of the carbon material in the methods described in JP-A-2000-182602 and JP-A-2001-15172, a bonding agent of the electrode is in direct contact with the lithium metal foil so that the bonding agent reacts with part of the lithium metal foil to form a highly resistant film.
Further, in the amorphous material sheet, the metal distribution inevitably becomes non-uniform in the microscopic scale resulting in the generation of the local concentration of the electric field. Because of these reasons, it is difficult to maintain the cycle performance at a higher level.
The following descriptions can be found in the above publications with respect to electrolytes. An electrolyte prepared by dissolving 0.4 weight part of LiBF4 and 12.1 weight part of LiPF6 into a mixed solvent composed of 65.5 weight part of diethyl carbonate and 22 weight part of ethylene carbonate is described in JP-A-2000-21392; an electrolyte prepared by dissolving 1 mol/liter of LiPF6 into a mixed solvent composed of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1 is described in JP-A-11(1999)-135120; an electrolyte prepared by dissolving 1 mol/liter of LiPF6 into a mixed solvent composed of ethylene carbonate and diethyl carbonate in a volume ratio of 2:8 is described in JP-A-10(1998)-21964; an electrolyte prepared by dissolving 0.4 g of LiBF4 and 12.1 g of LiPF6 into a mixed solvent composed of 65.3 g of diethyl carbonate and 22.2 g of ethylene carbonate is described in JP-A-2000-182602; and an electrolyte prepared by dissolving 0.4 g of LiBF4 and 12.1 g of LiPF6 into a mixed solvent composed of 65.3 g of diethyl carbonate and 22.2 g of ethylene carbonate followed by further dissolution of an adding agent such as 1,2-bis(ethoxycarbonyl)-1,2-dimethylhydrazine is described in JP-A-2001-15172. These electrolytes are described in the respective examples of the publications. Further, various solvents are cited and described to be used as an electrolyte in the bodies of the specifications.
However, the detailed review with respect to the optimum value and range regarding the solvent composition of the electrolyte, the volume ratio for mixing and the lithium salt concentration is not provided.